Infrared Analysis: Providing reliable & accurate biodiesel blend measurements

The easiest way to measure how much biodiesel is in diesel fuel is with infrared technology. That is why the American Society for Testing and Materials (ASTM) methods in the United States and the European Norm (EN) method in Europe use infrared measurements for biodiesel determination. With biodiesel production levels currently close to nine million gallons, and expected to reach 1.28 million by 2013, more biodiesel will ultimately be blended into our diesel fuel.

When biodiesel is blended into diesel, the “B” part relates to the percent concentration. For example a B20 is a 20% biodiesel and an 80% diesel blend. As many engine warranties are void if a higher blend is used than suggested by the manufacturer, a distributor can risk a lawsuit if they deliver a blend other than what is marked on the label.

A concern for those measuring biodiesel in diesel is whether the blend result will change if it was produced from a different source (such as from canola oil instead of soy oil, for example). Knowing more about how mid-infrared analysis detects biodiesel and the chemical structure of the feedstock oils will show why infrared is a convenient and accurate tool for measuring percent biodiesel in diesel.

Infrared measurement of biodiesel
Biodiesel in the US is usually a fatty acid methyl ester (FAME). FAME has an infrared signature that’s unique from diesel at the carbonyl band (5.73 micrometers or 1745cm-1). Carbonyl infrared absorbance occurs due to the stretching vibration of the carbon-oxygen double bond (C=O). At this infrared wavelength specific to biodiesel, the intensity of the absorbance increases as the concentration of biodiesel increases. Calibration standards of different biodiesel blends are used to correlate the infrared absorbance to a percent concentration.

Feedstock used for biodiesel production
The current increase in production has opened the doors to a number of potential biodiesel feedstock sources from algae and jatropha to grease from food production or municipal waste. The primary difference between the fatty acid esters in oils from various feedstocks is the length of the hydrocarbon chains, and the number and position of the C=C bonds (carbon-carbon double bond).

Most feedstocks such as soy, canola, and yellow grease (or waste vegetable oil-WVO) have chain lengths between C16 and C22, with C18 predominating. The chain lengths and their position affect cold-flow properties such as CFPP (Cold Filter Plug Point) and cloud point. Either of these parameters can cause a fuel filter to plug if it’s not correct for the climate.

Different feedstocks and infrared measurements                 
Table 1 shows the results from a B20 blend with five different feedstocks, measured with a Biodiesel Blend Analyzer. As indicated in the B20 column, most of the feedstocks are measured quite easily by infrared analysis, with the exception of coconut oil.

Table 1. Biodiesel feedstock comparison

The third column of the table shows the average molecular weight of the FAME (fatty acid methyl ester). Notice that the coconut oil is significantly different from the other oils, with a molecular weight of 180. According to the molecular weight the coconut-based FAME should give a response of about 26.7% (see sidebar). Any infrared analyzer can be calibrated specifically for a coconut oil, so the concentration measurement will be correct in spite of the molecular weight difference. Conveniently, the biodiesel made from coconut oil doesn’t perform well in cold or temperate climates, so its use is restricted to more tropical parts of the world.

Since mid-infrared measurements are compatible for a majority of the biodiesel feedstocks, it’s a simple and reliable analytical technique for checking biodiesel blend. The technology lends itself to compact and easy-to-use analyzers that enable petroleum terminals, fuel distributors, fleet operators, and regulatory agencies to make quick, onsite or laboratory measurements with little or no technical training. An accurate measurement is crucial to avoid over-blending for engines that could have compatibility issues.

For the math minded folks
Consider the following: (240 ÷ 180) x 20 = 26.7% (The average molecular weight of the methyl esters from the other feedstocks, divided by the average molecular weight of the methyl esters from coconut oil, times the biodiesel concentration.) 

Note: each type of oil will yield a mixture of esters, so there is no single molecular weight for the product from coconut oil.

~

Table 1 courtesy of “Chemistry of Organic Compounds, 3^rd Edition;” Carl R Noller, WB Saunders; Philadelphia (1965).

Sandra Rintoul is the president of Wilks Enterprise, Inc.

Wilks Enterprise, Inc.
www.wilksir.com